In recent years, composite fuel, which is mixture of gasoline and alcohol such as bioethanol, is widely recognized as fuel excellent in reduction in environmental pollution when used for an internal combustion engine. For example, JP-A-2007-278121 proposes a fuel supply system for selectively supplying composite fuel including main-fuel, which is high in alcohol concentration, and sub-fuel, which is low in alcohol concentration, into each cylinder of an internal combustion engine. Specifically, in the fuel supply system of JP-A-2007-278121, a fuel injection valve is provided near an intake port of each cylinder injects main-fuel, and an injection nozzle is provided at the upstream of the intake port of each cylinder injects sub-fuel. In the present structure, a duty-controlled valve controls sub-fuel, which is supplied to the injection nozzle and injected from the injection nozzle, when the internal combustion engine is started under a low temperature state, for example. However in the fuel supply system of JP-A-2007-278121, the injection nozzle is used for supplying sub-fuel being low in alcohol concentration. In the present structure, sub-fuel may not be sufficiently atomized and sprayed, and consequently the internal combustion engine may not be steadily started. Furthermore, in the fuel supply system of JP-A-2007-278121, main-fuel being high in alcohol concentration is regularly supplied to the fuel injection valve so as to reduce environmental pollution. In the present structure, deteriorated component of main-fuel may be easily cause deposit on the tip end of the fuel injection valve. When deposit accumulates on the tip end of the fuel injection valve, quantity of fuel injected from the fuel injection valve may be reduced, and consequently durability of the fuel injection valve may be reduced. Therefore, an occupant of the vehicle needs to perform a periodical maintenance operation such as supplying sub-fuel to the fuel injection valve so as to remove deposit from the fuel injection valve.